1. Field of the Invention
This invention relates to the protection of semiconductor devices from high temperature breakdown due to negative temperature coefficient (NTC) effects, in particular, it relates to the inclusion of positive temperature coefficient (PTC) material in a selective way to compensate for such NTC effects.
2. THE PRIOR ART
Gate-turn-off thyristors (GTO thryistors) for higher currents can be easily thermally destroyed during the turn-off process. If the turn-off in a part of the branched cathode structure is impeded, the forward current still flowing is conducted preferably in this location or part of the cathode. The resulting temperature increase leads to a reduction of the turn-off gain and of the forward voltage at the respective point or location which results in a further concentration of the current at the points of elevated temperature. Both effects lead to an increase of the turn-off power loss and thus to a further temperature increase. What can then happen is that the available turn-off gate current, at the respective point or location no longer suffices to interrupt the current flow. If the temperature of the semiconductor structure rises to about 130.degree. C., interrupting the current, when higher voltages are applied, is no longer possible. This is true since the forward biasing capability is increasingly lost at temperatures starting at about 125.degree. C. and rising about this valve.
If the point or location that experiences the increased temperature is small in size, this can finally lead to the destruction of the semiconductor structure because of the resulting high current density.
Thyristors, semiconductor rectifiers, and transistors can also be thermally destroyed by overly high forward current densities or surge current loads, which can cause local melting of the silicon or of adjoining silicon-metal eutectics. It is assumed that the destructive mechanism at work involves the fact that, the temperature first reaches such high values, as a result of the power loss, that the intrinsic concentration in this heated area rises to the order of the injected charge carrier concentration, and thus contributes substantially to the current in this area. In this way, the temperature dependence of the resistance of the semiconductor element changes from the usual increase with rising temperature to the abnormal decrease with rising temperature. In other words, the semiconductor element changes from the PTC resistance range to the NTC resistance range. The inversion temperature is generally between 500.degree. and 650.degree. C. in silicon power rectifiers and thyristors. In the NTC resistance range, a slight current increase leads to a further local current rise, due to the reduction of the resistance as a result of the temperature increase. With a given total current, this results in a current constriction, which can lead to the thermal destruction of the semiconductor element. This is enhanced or caused by inhomogeneities in the semi-conductor element material, which cause the current density to be increased at certain points of the semiconductor element. Such points include those where the carrier life is longer than normal and the like. Even if the semiconductor material does not melt, the semiconductor element can still become brittle and inoperative under repeated load, due to the high mechanical stresses caused by the inhomogeneity of the temperature.